1. Technical Field
The present invention relates to automated, sludge treatment processes and apparatus for converting sludge of widely varying high level, liquid content, such as raw sewage sludge, into a uniform, low moisture, pelletizable sludge product using combined chemical, mechanical and heat treatment steps.
2. Background Art
One of the most significant disposal problems facing modern civilization is how to economically handle the large amounts of sewage sludge produced daily without polluting the environment. This problem is compounded by the fact that sewage sludge is not produced uniformly, either across different sewage treatment plants, or at the same plant across time. Typically, sludge has been disposed of by composting, agricultural land application and/or landfilling. The largest cost component of these disposal options is transportation of the sludge, which is mostly comprised of water.
Attempts to solve the problem of high transportation costs have centered around various techniques for dewatering the raw sewage sludge before transportation. As described in U.S. Pat. No. 3,342,731 to Baumann, conventional dewatering steps involved the use of large area drying beds. As cities have grown and space has become more scarce, smaller-sized apparatus such as screening, heat drying and incineration have been used. To increase the effectiveness of these space saving techniques, flocculants, such as salts of iron and aluminum and organic polyelectrolytes, have been used but have been found disadvantageous because of the large quantity of chemicals often required. As disclosed n U.S. Pat. No. 3,342,731, recirculation of some of the processed sludge, in the form of incinerated residue, will greatly reduce the required quantity of flocculation promoting chemical. Another advantage of recirculation is to promote uniformity in the processed sludge as disclosed by Putnam in U.S. Pat. No. 1,915,240, page 2, lines 75 and 76.
One of the primary purposes for which heat processed sludge can be economically used is as a fertilizer. However, to be marketable as a fertilizer, heat processed sludge is preferably formed into pellets as disclosed in U.S. Pat. No. 3,025,151 to Berg et al. Pellet formation requires uniform sludge preferably having a relatively constant solids content.
One known type of apparatus and process for forming raw sewage sludge into pelletized sludge suitable for fertilizer is disclosed in U.S. Pat. No. 3,963,471 to Hampton. An integrated and automated system is disclosed in this patent including a dewatering phase, a drying phase and an air solids separation phase. In the dewatering phase, chemicals are mixed with liquid sludge supplied from a preliminary storage tank to a dewatering press to form a dewatered sludge cake. Drying takes place in the next phase by means of the addition of fuel to the sludge and ignition of the fuel in the dryer. Hampton states that it is necessary to recirculate hot dried sludge product from the output back to the input of the dryer for mixture with moist dewatered sludge cake entering the dryer to enable drying temperatures to be low enough to prevent formation of clinkers and the ignition of organic material being fed to the dryer. Hampton also states that the percentage of the final product recirculated also determines the size of the pellets produced with smaller pellets being formed as the percentage of recirculated product increases. Hampton discloses a control circuit for automatic delayed start up and shut down, but does not disclose such a system for application to other types of automated sludge processing systems.
Other forms of treating organic waste includes injecting heated gas into the waste while agitating the waste as illustrated in U.S. Pat. No. 3,676,074 to Shibayama et al. This approach, however, creates substantial waste gas having particles entrained therein. One method of eliminating the problem of large quantities of waste gases is to use an indirect heating method and apparatus as disclosed by U.S. Pat. Nos. 3,800,865 to Onarheim et al and 4,660,628 to Solberg, wherein a heat transfer fluid is used to heat plural hollow disks mounted for rotation with a hollow hub. The material to be dried passes through the space between the exterior of the disks and a surrounding housing but the heat transfer fluid is not allowed to come into contact with material being dried. This approach minimizes waste gas and, as noted in advertising material produced by Stord Bartz, assignee of U.S. Pat. Nos. 3,800,865 and 4,660,628, the type of disclosed indirect heater may be used to dry sludge without requiring sludge recirulation.
Others have recognized the advantages of indirect heat in reducing the quantity of waste gas as disclosed in a report of the U.S. Environmental Protection Agency dated September 1979, page 10-4, entitled "Process Design Manual for Sludge Treatment and Disposal".
While the prior art suggests a number of alternative approaches, many of which require recirculation of processed sludge in order to achieve uniformity and allow pelletization, no disclosure exists of an automated, non-polluting process or apparatus for mechanically dewatering and heat drying liquid sewage sludge of widely varying solids content to form a pelletizable dried sludge of predetermined uniform solids content without requiring recycling of previously heat processed sludge.